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Message-ID: <20120315053011.GK5728@port70.net>
Date: Thu, 15 Mar 2012 06:30:11 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: correctly rounded sqrt
* Rich Felker <dalias@...ifal.cx> [2012-03-15 01:07:48 -0400]:
> Here's the asm version based on fpu status word. I'm not sure if it's
> safe for non-default rounding modes but otherwise it should be
> correct.
up,down,zero rounding should be ok
we change
..xxx|10000..
into one of
..xxx|10100..
..xxx|01100..
and both of them round the same way with ru,rd,rz
> .global sqrt
> .type sqrt,@function
> sqrt: fldl 4(%esp)
> fsqrt
> fstsw %ax
> sub $12,%esp
> fld %st(0)
> fstpt (%esp)
> mov (%esp),%ecx
> and $0x7ff,%ecx
> cmp $0x400,%ecx
> jnz 1f
> and $0x200,%eax
> sub $0x100,%eax
> sub %eax,(%esp)
here you modify the return value even if it was nan
eg c99 F.9 recommends no nan modifications
if ((y & 0x7fff<<64) == 0x7fff<<64) ...
> fstp %st(0)
> fldt (%esp)
> 1: add $12,%esp
> fstpl 4(%esp)
> fldl 4(%esp)
> ret
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